


Birthday Puzzle

by SoloShikigami



Category: Undertale (Video Game)
Genre: Alternate Universe, Alternate Universe - Underfell, Alternate Universe - Underswap, Birthday, Fluff, Fontcest, M/M, Underfell Papyrus, Underswap Sans
Language: English
Status: Completed
Published: 2016-05-07
Updated: 2016-05-07
Packaged: 2018-06-07 00:39:46
Rating: Teen And Up Audiences
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 2,573
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/6777631
Author URL: https://archiveofourown.org/users/SoloShikigami/pseuds/SoloShikigami
Summary: <blockquote class="userstuff">
              <p>It's Sans' Birthday and he has been given a challenge... To solve a puzzle to receive his gift!<br/>Nothing the Magnificent Sans, Puzzle Master can't handle!<br/>(Not heavy fontcest, cute, sweet fluff)</p>
            </blockquote>





	Birthday Puzzle

**Author's Note:**

  * For [BattrelaRage](https://archiveofourown.org/users/BattrelaRage/gifts).



> My half of an art trade with my friend, Battre-la-Rage! Here is a link to their half of the trade:  
> [SFW Cute Pic of UT!Pap and US!Sans trading clothes because, you know, besties!](http://battre-la-rage.tumblr.com/post/144006224873/trade-with-lovely-soloshikigami-she-wanted-ussans)

            Sans woke with stars in his eyes. He quickly hopped out of his bed and got dressed. He made sure his battle body was super polished, his bandana had a bit of extra crispness to its appearance, and his gloves were super duper clean!

            "Mweh heh heh!" he snickered, posing in the mirror. "Today is going to be amazing! It's not everyday someone as 'Sansational' as me has a birthday!"

            He left his room with a skip in his step and went downstairs. He only half-expected to see his brother in the kitchen, since he sometimes made the effort to have some kind of breakfast ready for his beloved shorter brother on his birthday. Sans wasn't too disappointed to find the kitchen empty, but he did notice something that didn't quite belong. A piece of paper was propped up by the coffee pot.

            "Good morning, Sans, and Happy Birthday!" the note began. "There is a trail of notes leading to your birthday present. Follow the clues of the puzzle and if you do not fail, you may claim your reward."

            "As a Puzzle Master, the Magnificent Sans will not fail!" Sans declared to no one.

            "Your first clue - 'You dislike grease, your best friend does too, there's not a Grillby's Restaurant in your world, but the one with eight legs will do. The ceiling is too high, too low is the floor, but the next clue you seek will be behind the hidden door."

            Sans blinked. That was the end of the note. He read it a couple of more times before he began to laugh.

            "This one is too easy," he said with amusement, starting for the door. "Papyrus from the Undertale world is my best friend, Muffet's is to us what Grillby's is to them, but the hidden door... I wonder what that means."

            Sans kept a hold of the note tightly as he left the house.

 

* ~ * ~ * ~ * ~ * ~ * ~ * ~ * ~ * ~ *

 

            Sans walked briskly, his arms swinging, grin wide, and waving to everyone he came across.

            "Hey, little buddy, you seem to be in a good mood today," the Nice Cream man said to him as he passed.

            "I'm always in a good mood! Besides, today is my birthday!" Sans replied.

            "That's awesome!" A wide grin spread across the cat's face. "Anything fun planned?"

            Sans held up his note. "I have a puzzle to solve!"

            "That's awesome! Here, have a free Nice Cream for the occasion, it may help you along on your puzzle."

            He reached into his cart and handed Sans the frozen treat.

            "Thanks! That's very kind of you!"

            "You're welcome, little blue buddy," he gave Sans a wink. "Happy Birthday."

            "Thanks again!"

            Sans waved as he ran off. He glanced at the treat, deciding to wait a little before eating it - it was cold enough that it wasn't going to melt anytime soon. He looked at the note again, committing the important words to memory before entering Muffet's to look for the next note.

 

** ONE HOUR LATER **

            "Sans, what are you doing behind my counter?" an annoyed Muffet asked, the top set of arms crossed her chest, the ones below had her hands on her hips, and the lower set was scribbling away on an inventory sheet.

            "Looking for a hidden door! I was given a puzzle for my birthday!" Sans said, waving the paper at Muffet.

            The spider monster sighed and she looked at the note. She scratched the back of her head in confusion.

            “Hidden door? I don’t have a hidden door anywh- oh my stars no,” she suddenly muttered.

            Sans tilted his head curiously. Muffet looked very, very annoyed and she slammed her clipboard down on the counter before storming towards the end of the bar right after shoving the note back into Sans’ hands. Sans followed silently and curiously and watched as Muffet stopped in front of a framed photo of herself and Napstaton – she didn’t look pleased in the picture, either, and the robot was giving an obnoxious grin and thumbs-up. She took the photo away and Sans’ eyes widened as she tapped along a few places and part of the wall came away. She pried her fingers into it and a little door opened. Her face and posture dropped and she sighed with a shake of her head.

            “What is it, Muffet?” Sans asked, growing concerned.

            “I hide a few bottles of honey in here in case your brother clears me out. Looks like he found it anyway,” she muttered. “I’m going to charge him triple for that.”

            Sans couldn’t help crowding Muffet a little bit, trying to peek into the recess in the wall.

            “Hey, uhm, I’m sorry, Muffet, but is there a note or something in there for me?” Sans asked.

            Muffet moved aside to let Sans check for himself. He nearly stuck his face inside the small space and he squeaked in delight when he saw a folded piece of paper was sitting in the corner. He plucked it out and unfolded it carefully, keeping his eagerness in check.

            “Congratulations Sans, and Happy Birthday, too! You solved your first puzzle, so here’s clue number two! Slip your way through Snowdin town, travel to Waterfall, use a seed bridge so you don’t drown, a flower will reveal all!”

            Sans almost rolled his eyes. “This one is too easy, too! Ah, but then again, it must be difficult to conjure a puzzle that a master like myself cannot solve!” he turned to Muffet who closed up her hidden space in the wall, still exasperated. “Thank you, Muffet! And, erm, sorry that Papyrus found your honey stash.”

            Muffet sighed, but then a smile spread across her face. “You know I could never stay mad at that lazybones for long.”

            “Yeah, me neither!”

            “Well, I must return to my inventory. Happy Birthday, Sans, stop by later and I will have a special spider donut just for you,” she bent and gave the little skeleton a kiss on the cheek. “Also, you should eat up that Nice Cream before it melts.”

            Sans’ stomach rumbled slightly as he remembered that he also left the house without eating breakfast. He grinned at the spider monster, thanked her again, and was on his way.

 

* ~ * ~ * ~ * ~ * ~ * ~ * ~ * ~ * ~ *

 

            Sans sat down in his and Papyrus’ favorite spot with a huff. He had been all over the caverns filled with echo flowers but he couldn’t seem to find the spot with the seed bridge that the clue alluded to. He finished off the rest of the Nice Cream and looked at the stick, which was always inscribed with something nice, hence the namesake of the treat.

            “Look behind you.”

            Sans’ eyes widened at the unusual message and whipped his head around, partially expecting danger. There was no danger, but something caught Sans’ eye and he stood up to look.

            One really did have to be looking for the small opening at the back of the cavern, or the gems that lit it had to reflect off the water just so. Sans’ trademark grin returned as he stood up, cackling victoriously.

            “The Magnificent Sans never fails! Now, where are those seeds?”

            It didn’t take long before Sans was ducking into the small opening and found there was a secret little room. The gems here glowed with a little more white, and there was a lone echo flower growing next to a very, very small pond of water.

            Sans walked up to it quietly, as if approaching a sleeping animal, and he carefully kneeled beside it, putting his gloved hands over his mouth. Anything he thought to say he kept to himself, for he was certain this echo flower held his next clue and he didn’t want to ruin it by accidentally giving it something else to echo.

            “Happy Birthday, Sans,” the echo flower whispered. “Wondering where the end of this trail will be? Come to Undyne’s Lab for Clue Number Three.”

            Sans sighed a little as he started out of the small room.

           “I knew lazybones would get the better of him,” Sans said, a little disappointed in the lack of challenge. “But, then again, echo flowers only repeat so much.”

            After he crawled through the small opening and made it back over the seed bridge, he headed in the direction of Hotland, the ever-present spring still in his step as whispers from the echo flowers followed him like a breeze.

 

            “You’ve got to be kidding me,” Sans muttered, his hands on his hips.

            On the door of Undyne’s lab was a bright blue envelope with his name written on the front in a shaky hand. Sans was hoping to have something challenging waiting for him, but he took the envelope down and hoped that whatever was written inside would lead him to more puzzles.

            “Happy Birthday, Sans! I’m sorry I am not here to greet you, as I was sure you would be heading this way sometime today, but the air conditioner in here broke and I could not repair it on my own. Don’t worry too much, I did call someone who knows how to fix it, but in the meantime, I’m going to be over at Alphys’ place. I hope you have a wonderful birthday! Maybe we can celebrate later! Love, Undyne.”

            Sans tried to smile, grateful for the note, but at the same time he was starting to feel a little disappointed and lonely; it occurred to him that he didn’t know where Papyrus was, ever since Undyne and Alphys started dating usually they spent their time together at Alphys’ house and preferred privacy, and as he went along it just seemed that fewer and fewer people were about.

            With a sigh, Sans decided to start walking home.

 

* ~ * ~ * ~ * ~ * ~ * ~ * ~ * ~ * ~ *

 

            “Hey! You’re Sans, right?!”

            Sans was just about to exit Waterfall when the voice caught his ear. He turned to see an armless monster in a purple striped shirt grinning up at him.

            “Yeah, hey, I’ve seen you; you live in Snowdin too, right?” Sans asked.

            “Yuh huh!” Monster Kid did a graceful backflip in excitement. “And you train with Alphys, don’cha?”

            “I do! She’s going to make me one of the Royal Guard someday!” Sans straightened his back, then he winced slightly because he had been slouching through most of his walk home.

            “You must be coming back from training, huh? She must work you out real hard!”

            Sans’ smile fell a little. “Well, no, it’s my birthday, so, no training today.”

            “It’s your birthday! That’s so AWESOME! Can I follow you home? Please, please, please?!?”

            Sans started walking. “I suppose, but why would you want to?”

            “I want to see where the Magnificent Sans lives!”

            Sans grinned. “It’s not too hard to find; it’s the house with the skull flag outside of it. Almost everyone in town knows that.”

            Monster Kid gave a shrug. “Still! It’s your birthday! You shouldn’t walk home by yourself.”

            Sans chuckled. “All right, sounds good to me.”

            Monster Kid mostly gushed about how cool Alphys was and how neat that Sans was training with her and ‘wow can I come watch you guys train one day!’ Sans answered his questions, chuckled at him now and then, but still, his soul felt heavier and heavier as they approached his house.

            “Well, here we are,” he smiled down at Monster Kid. “Wanna come in? We’ll call your parents and let them know you’re with me.”

            “Sure!”

            Sans opened the door and frowned at how dark it was inside of his house.

            “What in the-?” he muttered.

            The door suddenly slammed shut behind him, making him jump with a yelp.

            “Monster Kid, you okay?” Sans said, keeping his voice steady as his soul pounded in his chest. He reached for the light switch, but it didn’t work. “What’s going on here?”

            Suddenly the light did come on.

            “ ** _SURPRISE!!!_** ”

            Sans’ jaw fell open; his living room was filled with all of his friends! The Royal Guard Dogs, Alphys and Undyne, Muffet, Grillby, many of the townsfolk, and there was his brother standing in the center with a huge grin on his face. Next to him were their twins from the other worlds – Undertale Sans and Papyrus as well as the pair from Underfell.

            “OH MY STARS YOU GUYS ARE THE BEST!” Sans shouted, running forward to be enveloped into a huge skeleton hugging pile. Even Underfell Sans grabbed his taller brother and yanked him into it. Then Sans let go and hugged them each individually, starting with his brother.

            “Pappy! That was the best present ever!” he said.

            “What do you mean?”

            “The puzzle! The surprise party!”

            “Ah. No, that wasn’t my idea. I just made sure everyone made it here.”

            “Oh. It must have been you guys!” Sans turned to the Undertale brothers and nearly tackled them in a hug.

            “Heh, wasn’t me, kid,” Undertale Sans said with a wink. “Though I did ask the Nice Cream guy for a favor.”

            “NYEH HEH HEH! I HELPED OUT WITH THE FIRST CLUES!” Undertale Papyrus declared. “BUT ALAS, THIS WASN’T MY IDEA.”

            “Oh,” Sans turned and grabbed Underfell Sans into a hug. The other growled but hugged him back. “It wasn’t you, was it?”

            “Nope.”

            Sans frowned at him.

            Underfell Sans chuckled. “I knew you’d be ‘puzzled.’”

            Before he could reprimand him for using a terrible pun and glare at his brother and Undertale Sans for laughing, party music filled the room.

            “Let’s hear it party people for SAAAAAANS!” Napstaton shouted into his microphone. Sans wondered how in the world he fit his DJ equipment inside. “Happy Birthday, baby!”

            There were accompanying shouts of well wishes and everyone started to dance as the beats filled the air.

            Sans also started dancing; he was surrounded by his friends, and he couldn’t be happier.

            Well, at least for a few minutes. He couldn’t help noticing that someone was missing.

 

* ~ * ~ * ~ * ~ * ~ * ~ * ~ * ~ * ~ *

 

            “I had the feeling I would find you out here,” Sans said, shutting the back door.

            Underfell Papyrus stood in the snow, his back to the house, staring through the pine trees that surrounded all of Snowdin. Sans approached him slowly.

            “You know, there aren’t any feral monsters hiding in the woods here,” he reminded the tall skeleton, remembering that from his world that was a constant threat.

            “I know. I was just waiting for you to figure out your puzzle.”

            Sans stood next to him and looked up, the other skeleton didn’t move, and Sans thought for a moment before speaking again.

            “I would have expected something more difficult from you,” Sans said. “And not so… sweet?”

            “I enlisted the help of all of our brothers, and you see the result,” he said stiffly. “The ultimate puzzle was to figure out who was behind the entire scheme.”

            “It seems that I have,” Sans grinned. “So what’s the reward for solving the puzzle?”

            Long, thin fingers reached under his chin and tilted his face up. Before he could think or react, Papyrus’ mouth clanked against his in a soft, gentle kiss. Sans’ eyes fluttered closed, the moment stretching on, he let out a small moan as hands settled on his shoulders and pulled him in slightly.

            “Will that suffice as your reward?” Papyrus murmured against his temple.

            “Only if the rest of you comes with it,” Sans murmured back with a pleased shudder.

            “Nyeh heh heh, of course.”

            As Papyrus returned his mouth to Sans’, the smaller skeleton decided that this was his best birthday ever.

**Author's Note:**

> I hope everyone enjoyed! Come check out my friend and I on Tumblr! Her link is above and mine is below (because kids like me should be burning in hell).
> 
> [SoloShikigami's Tumblr](https://www.tumblr.com/blog/soloshikigami)


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